cli: Only escape VPN banner if it's present

If the connection is in ACTIVATING state, the banner is still NULL.
vpn: Propagate daemon exec error correctly

NetworkManager[31624]: <warn>  VPN service 'openvpn': could not launch the VPN service. error: (8) Failed to execute child process "/usr/local/libexec/nm-openvpn-service" (No such file or directory).
**
NetworkManager:ERROR:nm-manager.c:3094:_activation_auth_done: assertion failed: (error)
libnm: Don't expect VPN connections to attach to the device
cli: Abort when given name of a non-existent connection for nmcli up

Even when ifname is present: nmcli c up name Nonexistent ifname wlan0
docs: fix build-from-tarball (again)
cli: Process secret agent request for a connection only if we know its path

If we're activating the device without knowing the connection in advance, defer
servicing the requests for the secrets until we set the path.

[lkundrak@fedora20-2 ~]$ nmcli --ask c ifname wlan0

(process:18405): libnm-CRITICAL **: nm_object_get_path: assertion 'NM_IS_OBJECT (object)' failed
Error: Connection activation failed: The device has no connections available.
[lkundrak@fedora20-2 ~]$
cli,vpn: merge branch 'lr/nm-1-0-fixes'

Fixes for various issues identified during 1.0 testing.

https://bugzilla.gnome.org/show_bug.cgi?id=740135